We are currently studying the origins of pulmonary hypertension in patients with chronic, diffuse interstitial lung disease (ILD), as well as the natural history of this group of diseases. Methodology involves the correlation of clinical data with assessments of lung function, pulmonary hemodynamics and anatomic disturbances (open lung biopsy). Data obtained thus far indicates that pulmonary hypertension in ILD stems from restriction of the bed by parenchymal disease, and that vasculitis rarely contributes to this process. Further, the relation between pulmonary arterial pulse pressure and stroke volume differs significantly from normal. This suggests that compliance of the large elastic pulmonary arteries is reduced in these patients. We propose to expand this study by characterizing the compliance of these vessels more directly. To this end instantaneous blood pressure- flow relationships in the main pulmonary artery will be assessed with velocity and pressure sensitive catheters and an angiographic estimate of the cross sectional diamter of the vessel. Pulmonary arterial input impedance will be calculated and submitted to harmonic analysis to characterize the distensibility of the central pulmonary arteries and the behavior of the small peripheral vessels.